1. Field of the Invention
The present invention relates to a temperature sensor having a temperature-sensing element, which has a temperature-sensing portion, such as a thermistor portion made of a thermistor material, or a platinum resistor, and is used to detect the temperature of an object of measurement. Particularly, the present invention relates to a temperature sensor preferably used to detect the temperature of an object of measurement, such as exhaust gas, in an environment exposed to vehicle vibration.
2. Description of the Related Art
Conventionally, a temperature sensor is known which has a temperature-sensing element and signal wires, the temperature-sensing element being composed of a thermistor portion and electrode wires extending from the thermistor portion where the electrode wires and the signal wires are welded (e.g., laser-welded) to one another in an overlapping condition. In the thus-configured temperature sensor, platinum or a platinum alloy (e.g., Pt—Rh alloy), having a high heat resistance and a low electrical resistance, is used to form the electrode wires of the temperature-sensing element. In view of heat resistance, strength, cost, etc., a stainless steel material or an INCONEL® material is used to form the signal wires.
A temperature sensor having such a configuration is used, for example, to detect the temperature of an object of measurement, such as exhaust gas, in an environment exposed to strong vibrations as in the case of the interior of a catalytic converter or an exhaust pipe of a vehicle. However, use in an environment exposed to such strong vibrations gives rise to a problem of breakage of the electrode wires of the temperature-sensing element due to the vibrations imparted to the temperature sensor. A technique to restrain breakage of the electrode wires is disclosed in Patent Document 1. Specifically, a dispersion-strengthened platinum material in which zirconia, yttria, or a like oxide is added to platinum or a platinum alloy is used to form the electrode wires of the temperature-sensing element.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2000-39364
3. Problems to be Solved by the Invention
As disclosed in Patent Document 1, application of a dispersion-strengthened platinum material, in which an oxide is added to the electrode wires, enhances the strength of the electrode wires. Thus, restrained breakage of the electrode wires can be expected. However, in a structure in which the electrode wires of the temperature-sensing element and the signal wires are welded to one another, zirconia, yttria, or a like oxide contained in a dispersion-strengthened platinum material does not melt at the time of welding of the wires, raising a problem of decreased strength of the weld zones. Specifically, the melting point of platinum is 1,770° C., and that of a platinum alloy is 2,000° C. or lower, whereas zirconia and yttria have very high melting points of 2,720° C. and 2,410° C., respectively. Accordingly, when the electrode wires are welded to the signal wires, such an oxide remains unmelted. This makes it difficult to strongly weld the electrode wires and the signal wires to one another. Thus, when a temperature sensor in which the electrode wires formed of an oxide-containing, dispersion-strengthened platinum material are welded to the signal wires is used in an environment exposed to strong vibrations, a new problem of fracture of a weld zone arises.
A temperature sensor is known which has an enclosing member made of metal, assuming the form of a closed-bottomed tube, and enclosing at least a temperature-sensing element and weld zones between electrode wires and signal wires. A ceramic filler is filled in at least a portion of an inner space enclosed by the enclosing member, the portion being located between one end of a temperature-sensing portion and a bottom portion of the enclosing member. When such a temperature sensor is rapidly cooled from a high temperature to a low temperature, a relatively large load is imposed on the weld zones for the following reason. When the temperature sensor is rapidly cooled from a high temperature to a low temperature, and thus the enclosing member begins to contract as a result of cooling, since the metallic enclosing member has a higher thermal expansion coefficient than the ceramic filler, the contraction of the internal filler fails to follow that of the enclosing member. Accordingly, the bottom portion of the enclosing member presses the temperature-sensing portion (temperature-sensing element) via the filler, so that a shearing stress is applied to the weld zones between the electrode wires and the signal wires.
In the case where the thus-configured temperature sensor employs a structure in which electrode wires formed of a dispersion-strengthened platinum material in which zirconia, yttria, or a like oxide is added are welded to signal wires to thereby form weld zones, since the strength of the weld zones is low as mentioned above, subjecting the temperature sensor to repeated rapid cooling from a high temperature to a low temperature raises another problem of likelihood of breakage (fracture) of wire at the weld zone(s).